Soft chemical dehydration route to carbon coating of metal oxides: Its application for spinel lithium manganate

On the basis of the dehydration reaction of carbohydrate molecules assisted by sulfuric acid, we have invented a novel soft chemical route to the carbon coating of metal oxides. As a representative, this technique has been applied to the carbon coating of spinel LiMn₂O₄ electrode material for lithium secondary batteries. Electron microscopy, elemental analysis, and N₂ adsorption-desorption isotherm measurements clearly demonstrate that porous carbon nanolayer with the thickness of ∼10-25 nm is successfully coated by the dehydration reaction of sucrose-adsorbed lithium manganese oxide at 90°C. According to power X-ray diffraction, Mn K-edge X-ray absorption, and micro-Raman spectroscopic analyses, the carbon coating does not modify the cubic spinel-type atomic arrangement of lithium manganate, while the coated carbon layer is composed of disordered amorphous carbon and polycyclic aromatic hydrocarbons. Of special importance is that the carbon coating can improve the electrode performance of spinel lithium manganate, which would be attributed to the increase of grain connectivity and/or to the protection of manganese oxide from chemical corrosion. This interpretation is further supported by the acid corrosion test showing that the dissolution of Mn ions in an acidic medium is remarkably depressed by the carbon coating. On the basis of the present experimental findings, we are able to conclude that the low-temperature dehydration method provides a novel, less energy-consuming route to the carbon coating of diverse metal oxides.